Fuel cells convert a fuel and an oxidizing agent into electricity, heat and water at two spatially separated electrodes. Typically in fuel cells, hydrogen or a hydrogen-rich gas is used as the fuel, and oxygen or air is used as the oxidizing agent.
The energy conversion process in fuel cells is distinguished by particularly high efficiency. For this reason, fuel cells are gaining increasing importance for mobile, stationary and portable applications.
The polymer electrolyte membrane fuel cell (PEMFC) and the direct methanol fuel cell (DMFC, a variation of the PEMFC, powered directly by methanol instead of hydrogen) are two common types of fuel cells that are used as energy converting devices. They are attractive choices because typically, they have compact designs, desirable power densities and high efficiencies.
The technology of fuel cells is well known to persons skilled in the art and is broadly described in the literature, see for example, K. Kordesch and G. Simader, “Fuel Cells and its Applications,” VCH Verlag Chemie, Weinheim (Germany) 1996. Nonetheless, in order to aid in the understanding of the present invention, in the following section, certain technical terms and phrases that are used in the present disclosure are described in greater detail:
A “catalyst-coated membrane” (hereinafter abbreviated “CCM”) consists of a polymer electrolyte membrane that is provided on both sides with a catalytically active layer. One of the layers takes the form of an anode for the oxidation of hydrogen and the other layer takes the form of a cathode for the reduction of oxygen. Because the CCM consists of three layers (anode catalyst layer, ionomer membrane and cathode catalyst layer), it is often referred to as a “three-layer MEA.” As outlined in this disclosure, a CCM may also contain one or more film layers for better protection, handling and sealing of the product.
“Gas diffusion layers” (“GDLs”), sometimes referred to as gas diffusion substrates or backings, are placed onto the anode and cathode layers of the CCM in order to bring the gaseous reaction media (e.g., hydrogen and air) to the catalytically active layers and, at the same time, to establish an electrical contact. GDLs usually consist of carbon-based substrates, such as carbon fiber paper or woven carbon fabric, which are highly porous and provide the reaction gases with good access to the electrodes. Furthermore, they are hydrophobic and permit removal of the product water from the fuel cell. Additionally, GDLs can be coated with a microlayer in order to improve the contact to the membrane. They can also be tailored specifically into anode-type GDLs or cathode-type GDLs, depending on into which side they are built in a given MEA. Furthermore, they can be coated with a catalyst layer and subsequently laminated to the ionomer membrane. These types of catalyst-coated GDLs are frequently referred to as “catalyst-coated backings” (abbreviated “CCBs”) or gas diffusion electrodes (“GDEs”).
A “membrane-electrode-assembly” (“five-layer MEA”) is the central component in a polymer-electrolyte-membrane (PEM) fuel cell and consists of five layers: the anode GDL; the anode catalyst layer; the ionomer membrane; the cathode catalyst layer; and the cathode GDL. A MEA can be manufactured by combining a CCM with two GDLs (on the anode and the cathode side) or, alternatively, by combining an ionomer membrane with two catalyst-coated backings (CCBs) at the anode and the cathode sides. In both cases, a five-layer MEA product is obtained. When the CCM contains one or more protective film layers integrated in the laminated assembly, the five-layer MEA in turn contains the protective film layer or layers as well.
The anode and cathode catalyst layers are comprised of electrocatalysts that catalyze the respective reactions (e.g., oxidation of hydrogen at the anode and reduction of oxygen at the cathode). Preferably, the metals of the platinum group of the periodic table are used as the catalytically active components, and for the most part, supported catalysts are used in which the catalytically active platinum group metals have been fixed in nano-sized particle form to the surface of a conductive support material. By way of example, carbon blacks with particle sizes of 10 to 100 nm and high electrical conductivity have proven to be suitable as support materials. In these applications, the average particle size of the platinum group metal is typically between about 1 and 10 nm.
The “polymer electrolyte membrane” consists of proton-conducting polymer materials. These materials are also referred to below as ionomer membranes. In ionomer membranes, tetrafluoroethylene-fluorovinyl-ether copolymer with sulfonic acid groups is preferably used. This material is marketed, for example, by E.I. DuPont under the trade name Nafion®. However, other, especially fluorine-free, ionomer materials such as sulfonated polyether ketones or aryl ketones or acid-doped polybenzimidazoles may also be used. Examples of materials that are suitable as ionomer materials are described by O. Savadogo in “Journal of New Materials for Electrochemical Systems” I, 47-66 (1998). For application in fuel cells, these membranes generally have a thickness between 10 and 200 μm.
Within fuels cells such and PEMFCs, one may stack several membrane-electrode-assemblies and bipolar plates in series to obtain the desired voltage output. As persons skilled in the art are aware, in fuel cell stack technology, sealing of components is an important issue. Generally, it is necessary to achieve a gas-tight sealing of these components (predominantly CCMs, MEAs and bipolar plates) against leakage to the environment and against intermixing of the reactants (hydrogen and oxygen/air). This gas-tight seal is essential for the safety of a PEMFC stack. (Lack of safety is a serious obstacle for the widespread introduction of fuel cell technology.) Thus, the quality and endurance of the seals and the materials used for them are of primary importance. For different stack architectures and operating conditions (such as pressure, temperature, fuel gases and lifetime required) different sealing concepts and technologies must be applied and developed. Furthermore, an appropriate sealing concept for CCMs and MEAs should also take into account an improvement for better protection and better handling of these products. Better handling and processing is particularly important in view of a large scale continuous production of CCMs and MEAs.
Various concepts and technologies for sealing of MEAs and CCMs are described in the prior art.
In U.S. Pat. No. 3,134,697, a sealing function is conventionally achieved by using pre-cut frames of a polymer material and placing these frames around the electrodes of the fuel cell between the membrane and the bipolar plates of the cell. However, this concept suffers from the high efforts needed for exact handling and positioning of the cell, membrane-electrode-assembly and gasket frames. Thus, there is no close contact between membrane and sealing.
EP 690 519 addresses the stabilization of the membrane in the inactive sealing region. It relates to an assembly consisting of at least one seal layer in a solid polymer ion exchange layer, wherein the seal layer or layers cover essentially only the region of the ion exchange layer that is to be sealed. According to this application, the sealing layer is made of polytetrafluoroethylene (PTFE) film having one surface coated and partially impregnated with the ionomer material.
A similar concept is pursued in WO 00/74160. This document describes a membrane electrode unit for fuel cells. There the membrane electrode unit comprises a reinforcing frame that is situated on the periphery and in the area of openings that are placed in the active portion of the membrane electrode unit and provided for guiding material or for installation. A reinforcing frame is formed by a hot melt type adhesive layer that is applied on both sides and is formed by at least one rigid plate.
All of these concepts described in the aforementioned references are based on sealing frames or layers that cover the peripheral membrane rim of the CCM/MEA and only stabilize this peripheral rim. However, depending on fuel cell operating parameters, frequently failures in the membrane material may occur at the interface between the active area and the sealing layers. Therefore, sufficient overlap is needed between the sealing/gasket layer, the active electrode layer and the ionomer membrane.
WO 00/74161 relates to a membrane-electrode-assembly provided for fuel cells or the like that comprises an ionomer membrane that is coated on both sides with electrodes. The sealing edge, which is configured on the outer periphery, is comprised of a hot melt adhesive whose hydrocarbon skeleton carries, at regular intervals, ionic or strong polar groups that enter into a surface interaction with the ionic groups of the membrane material and thus provides for good adhesive effect of the hot melt type adhesive to the polymer electrolyte membrane. The thermoplastic sealing made of hot melt type adhesive extends on both sides over the edge section of the membrane. Unfortunately, associated with this method are high production costs, as well as costs for the application form of the hot-melt adhesive. Furthermore, its long-term stability is not proven; various components (such as hardeners, defoamers and other additives) may be leached out during operation and may cause deterioration of the MEA.
WO 00/10216 describes a membrane electrode gasket assembly (“MEGA”) having a gasket and a sub-gasket to seal the MEA and to protect it from possible edge failures. The gasket material typically consists of expanded polytetrafluoroethylene (e-PTFE), soaked with a solution of ionomer for better adhesion. The sub-gasket is disposed over a peripheral portion of an electrode, which is applied to a central portion of an ionomer membrane. In the examples given in WO 00/10216, a simultaneous overlapping of the sub-gasket with the electrode portion and the non-coated ionomer membrane portion is not disclosed.
A different concept is suggested in U.S. Pat. No. 5,176,966. According to that patent, seals are formed by impregnating the layers of porous electroconductive sheet material of the membrane electrode assembly with a sealant material that generally circumscribes the fluid passage openings and the electrochemical active portions of the assembly.
Another disclosure, WO 98/33225, is directed to a sealing that penetrates an edge of at least one gas diffusion electrode (GDE) whereby the pores of the electrodes are filled. In that disclosure, the sealing is bonded to the membrane where the sealing penetrates the electrodes and comes into contact with the membrane and is also bonded to the peripheral face of the membrane. Both surfaces of the membrane are essentially completely covered by the electrodes.
The latter two concepts, which are based on impregnation of gas diffusion electrodes (GDEs) with a sealant material, suffer from the complexity of the impregnation process. Even small deviations of the process parameters strongly affect quality of sealing and the smoothness of the contact surface of the sealed area. Consequently, it is very difficult to obtain gas-tight seals.
In light of the shortcomings of the prior art, the present invention is directed to an improved catalyst-coated membrane that avoids the described disadvantages of the state of the art. In particular, the present invention provides a catalyst-coated membrane embracing one or more protective film layers that offers the following advantages: (i) improved mechanical stability; (ii) improved protection against membrane damage; and (iii) improved handling properties in cell/stack assembly. The present invention also provides an improved membrane-electrode-assembly (MEA) that offers the above-mentioned advantages. Finally, a process for manufacture of these improved products is outlined.